Here's a board I've been meaning to get out for a while to complement the range of nRF24L01+ related boards I sell through my website. (I will probably also sell it on Tindie.) They haven't come back from the fab yet, so this is just a board rendering from Eagle:

The new variant basically substitutes the ATmega 328p chip with a Pro Mini 328 board. (Given the price of AVR chips at the moment, it seemed like a good time finally get around to this!)

It has the same Arduino shield header layout, plus provision for the second 0.1" grid aligned breadboard/protoboard compatible set of headers.

It also features, of course, built-in nRf24L01+ headers. There are actually two sets of nRf24L01+ headers, one connected up to the hardware SPI pins, and the other connected up to an alternate set of pins for software (bit-banged) SPI. (The alternative is just to provide flexibility in situations where the HW SPI pins are unavailable for whatever reason.)

The voltage regulator is a big old TO-220 package LD1117 -- wasteful in terms of real estate, perhaps, but keeps the assembly entirely as easy through-hole soldering for beginners. The remainder of the board real estate is given over to prototyping area.

No USB by default, although there is room to mount a compact USB to Serial CP2102 based module on the protoyping area if desired. This all gives the small board a lot of flexibility.

The Pro Mini variant is bigger board, about 5x7cm (very close to Uno sized), whereas the 328p dev board is about 5x5cm (really, just the Arduino shield header footprint, no bigger).

The newer board had to be expanded so the shield area was clear, as the Pro Mini sits with a higher profile than the socketed 328p chip. But the advantage is there is little more prototyping area as a result.

There is also an extra header to optionally break out A6 and A7, if the version of Pro Mini has those available. (There are actually a few variations on the Pro Mini design floating around these days -- I've tried to design the board so it will cover all the variations I'm aware of. If there's a variation out there I've missed, and you think it may not work with this layout, please let me know.)

Provision for a female barrel jack connector has been added.

I've also just finished a similar design for a PJRC Teensy 3.x board, which I'm waiting to come back from the fab.

I've been toying with the idea of doing a 1284p version, to fill out the "Mega" space for nRF24L01+ enabled dev boards, but I am not convinced it's really necessary given the price/performance of the Teensy 3.x. My feeling is if your project outgrows a 328, you might as well skip the Mega and just go straight to the Teensy 3.x. May still do a 1284p one anyway, "just because". I really suspect it's been squeezed out by the Teensy, however. Just too much bang for buck!

Anyway, while the original raison de etre for these boards was to provide easy built-in support for nRF24L01+ module connectivity across a range of processors, the aim was that the design should be general enough that they could be useful and economical deployment boards even if the nRF24L01+ radio modules were not required.

Actually, a possible neat solution would be to use a 5V power supply for Vin, so that you can put 5V onto the 5V pin on the power header without any additional cost. (That's what the VIn/5V jumper header is for. Jumper it if using 5V Vin, disconnect if not.).

Running a 5V Vin on a 5V Pro Mini would will mean it running at a lower Vcc than 5V, though; how much lower depending on what onboard regulator is used on the particular Pro Mini. Just depends on the characteristics of the linear regulator used.

But if it is at or above 3.78V, it will still be in spec to run at 16MHz.

Note that the 328p chips on a 5V Pro Mini and 3v3 Pro Mini are identical. It is only the onboard regulators and crystal frequencies that vary. So there is nothing wrong with running a 328p clocked at 16MHz at, say, 3.8V.

I was thinking same basic idea with the nRF24L01+ headers etc., but maybe a longer board (about 5x10cm, i.e., Mega/Due size), with the extra room for the chip itself, the bigger additional breakout header, and the balance going to some extra prototyping space. I expect I will end up doing it just to satisfy curiosity, but I'd be quite surprised if turned out to be very popular, as my suspicion is that for the few extra bucks the Teensy 3.x would just represent a much better value proposition all around!